1. Field of the Invention
This invention is directed to a lithography projection system that can be used to transfer a pattern on a mask reticle to a wafer substrate. For example, the system can be used to pattern a resist layer in a process for forming an integrated circuit device or other features on a wafer substrate.
2. Description of the Related Art
The smallest feature size that can be formed on a substrate with a lithography system is proportional to the wavelength of light used in the system. The current state-of-the-art systems use light wavelengths in the deep ultraviolet (DUV) range on the order of hundreds of nanometers, and are thus inherently capable of forming features sizes with smallest dimensions on the order of several tenths of microns. With the ongoing demand in integrated circuit and other industries for smaller features sizes, it would be desirable to overcome the inherent limitations of existing lithography projection systems, and to provide a system capable of producing feature sizes on the order of 0.1 micron or less. This would require using light wavelengths in the extreme ultraviolet (EUV) range.
Of some relevance to this invention are four-mirror catoptric systems that have been proposed for use in the performance of DUV lithography. Examples of these kinds of systems are disclosed in Four-mirror imaging system (magnification+1/5) for ArF excimer laser lithography, Cheon Seog Rim et al., Optical and Quantum Electronics, Vol. 27 (1995), pages 319-325, Improved four-mirror optical system for deep-ultraviolet submicrometer lithography, Jong Tae Kim et al., Optical Engineering, Vol. 32, No. 3, March 1993, pages 536-540, Cassegrainian-inverse Cassegrainian four-aspherical mirror system (magnification=+1) derived from the solution of all zero third-order aberrations and suitable for deep-ultraviolet optical lithography, Young Min Cho et al., Optical Engineering, Vol. 33, No. 7, July 1994, pages 2480-2486, and Four-mirror optical system for UV submicrometer lithography, Sung Chan Park et al., Optical Engineering, Vol. 30, No. 7, July 1991, pages 1023-1027. Although these systems are asserted to be useful in the performance of DUV lithography in the above-listed publications, they are specially configured for use at DUV wavelengths, and as such are incapable of functioning effectively at EUV or shorter wavelengths to achieve integrated device feature sizes on the order of 0.1 microns or less. It would be desirable to overcome these disadvantages of previous lithography projection systems.